3.1 The calculations should be based on moulded
lines and include large appendages such as shaft bosses, skegs and
bow thrusters.
3.2 The metacentric heights (GM), stability levers
(GZ) and the centre of gravity positions (KG) for judging the final
survival conditions should be calculated by the constant displacement
(lost buoyancy) method.
3.3 The calculations should be done for the ship
freely trimming.
3.4 Only computer calculations acceptable to the
Administration should be used.
3.5 Where the assumed damage causes the ship to
trim by the stern, the ship in the intact condition should be assumed
to have the largest allowable trim by the stern, consistent with operational
requirements.
3.6 Where the assumed damage causes the ship to
trim by the bow the ship in the intact condition should be assumed
to have the largest allowable trim by the bow, consistent with operational
requirements.
3.7 Lesser extent of damage should be taken into
account only where indicated by the presence of subdivision extending
into the maximum extent of damage, e.g. double-bottom tanks, side
ballast tanks, side cargo tanks, fuel tanks and void spaces. However,
the following should be given attention:
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.1 “Lesser extent" means the reduction of
any one of the three maximum dimensions of damage singly or in combination
and also the assessment of the effect of damage affecting any combination
of compartments with the maximum extent of damage.
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.2 Where any damage involves the release of very
heavy cargo liquid, then heel to the intact side of the ship may take
place. In such cases the effect of lesser vertical extent of the damage
above the level of the tank top may result in the larger angle of
heel, since otherwise the effect of cargo loss may be compensated
by flood water entering the double-bottom tanks on the damage side.
3.8 The number of calculations required to show
compliance with survival requirements should be that necessary to
obtain sufficient data for the loading manual and should be such that
all loading conditions indicated in 1 can be covered, i.e. no additional
calculations should be necessary once the series of calculations has
been executed.
3.9 Calculations to determine the displacement,
trim and the vertical position of the centre of gravity should be
performed for each operational loading condition. The vertical position
of the centre of gravity should be corrected for free surface effects.
One method would be to construct graphs showing the free surface moments
of the criterion angle, for all filling levels at a specific gravity
of one. The free surface moments for all tanks can then be taken from
the graphs and be multiplied by the cargo specific gravity.
3.10 In calculating the effect of free surface
of consumable liquids it is to be assumed that, for each type of liquid,
at least one transverse pair or a single centreline tank has maximum
free surface, and the tank or combination of tanks to be taken into
account are to be those where the effect of free surfaces is the greatest;
in each tank the centre of gravity of the contents is to be taken
at the centre of volume of the tank. The remaining tanks are to be
assumed either completely empty or completely filled, and the distribution
of consumable liquids among these tanks is to be such as to obtain
the greatest possible height above the keel for the centre of gravity.
3.11 To take account of the presence of structure
within cargo compartments, a permeability of 0.95 should be assumed
as stated in the Codes. Where, in particular cases such as the cargo
tanks of gas carriers, this assumption would lead to a significant
discrepancy in cargo tank volume, it is preferable to calculate the
permeability taking into account actual tank structure, the volume
of tank insulation should then be calculated separately and an appropriate
permeability applied.
3.12 Attention should be paid to the possibility
of progressive flooding through deck cargo pipes and common cargo
tank ventilation pipes, if these are immersed at large angles of heel
after damage. The possibility of progressive flooding through ballast
piping passing through the assumed extent of damage, where positive
action valves are not fitted to the ballast system at the open ends
of the pipes in the tanks served, should be considered. Where remote
control systems are fitted to ballast valves and these controls pass
through the assumed extent of damage then the effect of damage to
the system should be considered to ensure that the valves would remain
closed in that event.
3.13 Where the ship is required to be capable
of sustaining bottom damage anywhere in its length (L), the following
method should be used when damage is assumed to occur in the vicinity
of the 0.3 L position from the forward perpendicular:
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.1 when applying the longitudinal extent of bottom
damage applicable to the foremost part of the ship, no part of the
damage should be assumed to extend abaft the 0.3 L position from the
forward perpendicular.
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.2 when applying the longitudinal extent of damage
applicable to the rest of the ship's length the damage should be assumed
to extend to a foremost limit including a point at 0.3 L minus 5 m
abaft the forward perpendicular.
3.14 In ships carrying liquefied gases, large
cargo tanks may be subdivided into the sections by centreline and
transverse bulkheads which are liquid-tight but which have openings
near the top of the tank. These openings would permit spillage of
cargo from one section of the cargo tank to another when the ship
is heeled where the tank is undamaged, or loss of cargo due to spillage
from sections of a damaged cargo tank. The effect of this spillage
should be taken into account in calculations and also in any calculation
of GM or KG for loading conditions where a “required GM' or
“allowable KG" curve is to be used.
3.15 In ships carrying liquefied gases, the ability
of longitudinal bulkheads fitted within cargo tanks to withstand the
unequal pressures due to flooding of one section of cargo tank should
only be considered in the final stage of flooding.
3.16 Where lubricating oil drain tanks fitted
below the main engine would be affected by the vertical extent of
bottom damage then flooding of the engine-room by way of the drain
tank and engine should be assumed to take place.
3.17 In ships with machinery spaces aft, the machinery
space and steering gear compartment should be regarded as being common
for damage purposes when any access is fitted in the after machinery
space bulkhead, unless a remotely operated sliding watertight door
is fitted, or the sill of the access openings fitted hinged watertight
doors which are to be kept closed at sea is at least 0.3 m above the
damage waterline and will not be submerged within the minimum range
of residual stability.
3.18 Where dry cargoes are carried at the same
time as bulk liquid cargoes which require compliance with the requirements
of the Codes then the permeability of the space carrying the dry cargo
is to be ascertained.
3.19 The harmonized regulations specify that no
account should be taken of cross-flooding arrangements to attain stipulated
limits of heel in the final state of equilibrium after damage. However,
compartments on the opposite sides of a ship could be regarded as
single compartments from the aspect of flooding if they were to be
linked by openings or ducts of sufficiently large area. In such cases
consideration should be given to the adequacy of tank air flow and
to the effect of free surface.